There are various systems for mobile phones, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, PCS (personal communications services) used in the U.S., and various systems using TDMA (time division multiple access) such as PDC (personal digital cellular) used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequency, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
To utilize a plurality of systems, a user should have a plurality of mobile phones each adapted for each system, or a small, lightweight mobile phone capable of communicating in plural systems. In the latter case, to make one mobile phone adaptable for a plurality of systems, the mobile phone should comprise parts for every system. For instance, in a system of transmitting signals, high-frequency circuit parts, such as filters for passing transmission signals of desired transmission frequencies, high-frequency switches for switching transmitting circuits and receiving circuits, and an antenna for receiving and emitting transmitting/receiving signals, are needed for every system. In a system of receiving signals, for instance, high-frequency circuit parts such as filters for passing received signals of desired frequencies among those passing the high-frequency switches are needed for every system. Accordingly, such mobile communications devices are not only expensive but also have unsuitably large volume and weight.
Small and lightweight high-frequency circuit parts adapted for a plurality of systems are thus demanded. For instance, JP 11-225088 A (EP 0921642 A3) discloses a dualband antenna switch module adapted for EGSM and DCS. FIG. 30 is a circuit block diagram showing a dualband antenna switch module adapted for the EGSM system (transmission frequency: 880 to 915 MHz, receiving frequency: 925 to 960 MHz) and the DCS system (transmission frequency: 1710 to 1785 MHz, receiving frequency: 1805 to 1880 MHz). A diplexer Dip branches an EGSM signal in 0.9 GHz band and a DCS signal in 1.8 GHz band, a first high-frequency switch SW1 switches connection to an EGSM transmission terminal (Tx) and an EGSM receiving terminal (Rx), a second high-frequency switch SW2 switches connection to a DCS transmission terminal (Tx) and a DCS receiving terminal (Rx). Lowpass filters LPF1, LPF2 reduce harmonic strain generated by a power amplifier on the transmission side.
JP 2000-165288 A (EP 0998035 A3) discloses a tripleband antenna switch module for mobile communications devices adapted for three systems of EGSM, DCS and PCS.
As a next generation mobile wireless system, the service of a W-CDMA (wide-band code division multiple access) system is starting, and it is expected to rapidly spread because of a higher data transmission rate, multichannel communications, etc. Accordingly, mobile wireless devices adapted for W-CDMA in addition to EGSM, DCS, PCS, etc. presently predominant among mobile phone communications systems have become necessary.
FIG. 31 is a circuit block diagram showing an antenna switch module adapted for three bands of EGSM, DCS and W-CDMA (transmission frequency: 1920 to 1980 MHz, receiving frequency: 2110 to 2170 MHz). A diplexer Dip branches signals in a frequency band of EGSM and signals in a frequency band of DCS/W-CDMA. Though it synthesizes signals in an opposite direction, only “branching” will be explained here for simplicity. A first high-frequency switch SW1 switches connections to an EGSM transmission terminal (Tx) and an EGSM receiving terminal (Rx), while the second high-frequency switch SW2 switches connections to a DCS transmission terminal (Tx), a DCS receiving terminal (Rx) and a transmitting/receiving terminal of W-CDMA. Lowpass filters LPF1, LPF2 reduce harmonic strain generated by power amplifiers on the transmission sides.
However, the above conventional technologies have the following problems.
(1) Breakdown of High-Frequency Parts by Electrostatic Surge
High-frequency parts such as pin diodes, FET switches, etc. used in the high-frequency switch circuits shown in FIGS. 30 and 31 are vulnerable to electrostatic charge. Particularly in the case of mobile phones, the above high-frequency parts are subjected to breakdown when electrostatic surge from human bodies is input into antennas. In addition, though an antenna switch module per se is not subjected to breakdown, circuits connected downstream of the antenna switch module, such as power amplifiers and low-noise amplifiers, etc. connected to transmission terminals and receiving terminals, respectively, of the antenna switch module are likely subjected to breakdown. Therefore, measures for electrostatic surge have been important.
As a technology for removing electrostatic surge, a circuit shown in FIG. 32(a), which is described in JP 2001-186047 A, has conventionally been known. This circuit comprises an inductor connected to a ground, which is connected to part of two diplexers. To protect high-frequency parts from electrostatic surge by this circuit, the inductor connected to the ground should be 5 nH or less. When an inductor of 5 nH or less is connected to an antenna top, however, it is difficult to secure matching in a wide band from 900 MHz to 1.8 GHz. Attenuation near 300 MHz is actually as small as 5 dB or less in this circuit, insufficient for the removal of electrostatic surge.
JP 2001-44883 A discloses a circuit shown in FIG. 32(b), in which an inductor connected to a ground and a capacitor are disposed in a signal line of each of an antenna terminal ANT, a transmission terminal Tx and a receiving terminal Rx. However, the insertion of an LC filter comprising an inductor L1 and a capacitor C1 in each of the antenna terminal, the transmission terminal and the receiving terminal not only prevents the miniaturization and cost reduction of a high-frequency switch, but also causes the deterioration of insertion loss. Attenuation near 300 MHz is actually as small as about 5 dB in this circuit, insufficient for the removal of electrostatic surge.
Though it may be considered to use varistor diodes and Zener diodes as electrostatic-surge-removing parts, it necessitates a large capacitance between the terminals, inevitably resulting in the deterioration of insertion loss when used in signal lines. Accordingly, the varistor diode and the Zener diode cannot be used to remove electrostatic surge in a high-frequency circuit comprising high-frequency switches.
(2) Generation of Harmonics (in Switch Circuit Comprising Pin Diode)
The circuit shown in FIG. 31 has a problem that harmonic strain is generated by a W-CDMA transmission signal passing through the second high-frequency switch SW2. It is generally known that when a high-power, high-frequency signal is input to a nonlinear device such as a pin diode and a GaAs switch, harmonic strain is generated. Particularly with a pin diode, harmonic strain generation in its OFF state is remarkable. The reasons therefor are clear from the V-I characteristics of the diode shown in FIG. 33. Namely, in an ON state, the diode is driven at an operation point having relatively good linearity by voltage Vc of the control voltage, so that the diode linearly responds to voltage change by high-frequency signals, resulting in less harmonic generation. On the other hand, the operation point of the diode is around V=0 in an OFF state, so that the diode unlinearly responds to voltage change by high-frequency signals, resulting in large harmonic generation.
FIG. 34 is a view showing an equivalent circuit of the tripleband antenna switch circuit shown in FIG. 31 adapted for EGSM, DCS and W-CDMA. Table 1 shows the ON/OFF states of pin diodes and control voltage in each operation mode. In the control voltage, “High” is desirably +1 V to +5 V, and “Low” is desirably −0.5 V to +0.5 V.
TABLE 1ON/OFF of pin diodes in circuit shownin FIG. 34 in each operation modeControl VoltageON/OFF States of Pin DiodesModeVC1VC2VC3D1D2D3D4D5D6EGSMHighLowLowONONOFFOFFOFFOFFTxDCS TxLowHighLowOFFOFFONONOFFOFFEGSMLowLowLowOFFOFFOFFOFFOFFOFFRxDCS RxLowLowHighOFFOFFOFFOFFONONW-LowLowLowOFFOFFOFFOFFOFFOFFCDMA
It is clear from above that the diodes D1, D2 in an ON state are connected to a path from the EGSM TX terminal to the antenna ANT in an EGSM transmission (Tx) mode, while the diodes D3, D4, D5, D6 in an OFF state are separate as a circuit, resulting in reduced harmonic generation.
In a DCS transmission (Tx) mode, too, diodes D3, D4 in an ON state are connected to a path from the DCS TX terminal to the antenna ANT, while the diodes D1, D2, D5, D6 in an OFF state are separate as a circuit, resulting in reduced harmonic generation.
In a W-CDMA mode, diodes D3, D4, D5, D6 in an OFF state are connected to a path from the W-CDMA terminal to the antenna ANT, and when a high-power signal is input from the W-CDMA terminal, a large harmonic signal is emitted from the antenna terminal ANT. This means that a signal that should not be emitted from an antenna of a mobile phone is emitted, a problem unavoidable in the conventional art.
(3) Miniaturization, Power Saving and Harmonic Generation (when FET Switch is Used)
The high-frequency switch module shown in FIG. 30 needs four pin diodes in total, and the high-frequency switch module shown in FIG. 31 needs six pin diodes in total, thereby preventing their miniaturization and power saving. Generally, a switch circuit comprising a GaAs switch has less power consumption than that comprising a pin diode.
U.S. Pat. No. 5,815,804 discloses, as shown in FIG. 35, an example comprising a diplexer Dip1 on a transmitting side for branching an EGSM transmission terminal (Tx) and a DCS transmission terminal (Tx), a diplexer Dip2 on a receiving side for branching an EGSM receiving terminal (Rx) and a DCS receiving terminal (Rx), and one FET switch such as a GaAs switch, etc. as a switch circuit SW for switching the diplexers Dip1 and Dip2. In this case, the miniaturization and the reduction of power consumption can be achieved than in a switch circuit comprising pin diodes. However, because the switch circuit SW is connected to the diplexer Dip1 on a transmitting side in an EGSM transmission mode, there is a problem that a DCS band signal supplied from the DCS transmission terminal (Tx) also passes. Though a power amplifier on the DCS side is set not to be operated in an EGSM transmission mode, a signal is slightly generated from the power amplifier on the DCS side because of oscillation by a second harmonic of the EGSM transmission signal and crosstalk with an amplifier on the EGSM. This phenomenon is particularly remarkable in the case of a dual power amplifier comprising two power amplifiers of EGSM and DCS in one package, so that a signal of about −15 dBm may be output from the power amplifier on the DCS side. Namely, a signal of 1.8 GHz corresponding to two times the frequency of the EGSM transmission band is input into the DCS transmission terminal in an EGSM transmission mode, and the diplexer Dip1 and the switch SW permit a signal of 1.8 GHz to pass through, posing the problem that second harmonic strain of EGSM transmission is emitted from the antenna. The second harmonic emitted from this antenna is desirably −36 dBm or less.
There is also an example comprising a diplexer Dip1 for branching an EGSM receiving terminal (Rx) and a DCS transmission terminal (Tx), and a diplexer Dip2 for branching an EGSM transmission terminal (Tx) and a DCS receiving terminal (Rx), a switch circuit SW for switching Dip1 and Dip2 being similarly constituted by one GaAs switch. There is basically a problem that a GaAs switch is more likely to generate harmonic strain than a circuit comprising pin diodes. Power of +36 dBm at most may be supplied to a GaAs switch particularly in EGSM transmission. Accordingly, to suppress second harmonic generation to −36 dBm or less, an EGSM transmission signal generated from the GaAs switch per se should have second harmonic of −72 dBc or less. However, it is difficult at present to obtain GaAs switches with such small harmonic generation. Though the reduction of harmonic generation from the GaAs switch can be easily achieved by increasing supply voltage, increased supply voltage in parts used in mobile phones needs the increased supply voltage of batteries.
In the case of a circuit comprising a GaAs switch directly switching transmitting and receiving signals of plural frequencies without using a diplexer, the GaAs switch is more vulnerable to breakdown by electrostatic surge than a pin diode. To protect the GaAs switch from electrostatic surge, the inductor connected to a ground should be 5 nH or less. However, when the inductor of 5 nH or less is connected to an antenna top, it is difficult to obtain matching in a wide band from 900 MHz to 1.8 GHz. Because of this, too, the conventional electrostatic-surge-removing circuit cannot be used on an antenna top.